English: Rhyodacite pumice from the late Holocene of Indonesia. (8.6 cm across at its widest)

Krakatoa (also spelled Krakatau) is a subduction zone volcano in Indonesia’s Sunda Strait between the large islands of Sumatra and Java. It had a catastrophic eruption in late August 1883, culminating in an enormous steam explosion as its magma chamber collapsed. The steam explosion generated a large tsunami, which wiped out many villages in surrounding areas. Tens of thousands were killed. The steam explosion was heard several thousand miles away, and the shock wave went around the entire globe. For some time after, Krakatoa ash high in the atmosphere resulted in gorgeous orangish-red sunsets around the world. An artist in Scandinavia was inspired by the Krakatoa sunsets to make the famous surrealistic painting “The Scream”.

Krakatoa’s eruption resulted in the generation of enormous volumes of ash and pumice. Sailors noticed floating Krakatoa pumice for months afterward, sometimes mixed with the bodies and bones of victims killed by the tsunami. The rock shown above is a pumice piece from Krakatoa’s August 1883 eruption - it floated in the Indian Ocean Basin for almost a year before washing ashore in eastern Africa. Most of Krakatoa’s erupted ash and pumice is rhyodacite. A small percentage of the erupted material is dacite and andesite.

As are all pumice samples, this rock consists of highly porous, finely vesiculated volcanic glass. The abundance of gas bubbles (frothy texture) makes the rock lightweight. Many of the vesicles (gas bubbles) are noticeably stretched.

Collection locality: Takwa Beach, southeastern shores of Manda Island, Lamu Archipelago, coastal Kenya, eastern Africa